BUG: external_free_convection_correlation

[DanielePlacidoPolito]

Describe the bug
Flag external_free_convection_correlation does not work and/or does not anything in sheet CONDUCTOR_input of input file conductor_definition.xlsx

To Reproduce
Steps to reproduce the behavior:

1. Go to input file conductor_definition.xlsx
2. Click on sheet CONDUCTOR_input
3. Scroll down to row 22
4. Change the value of the flag
5. Run the program
6. See error

Expected behavior
The software works fine and correctly evaluate the natural convective heat transfer coefficient

Desktop (please complete the following information):

• OS: Windows 10